The present disclosure relates to a design method of a plasmonic crystal. Particularly, the present disclosure relates to a design method of the plasmonic crystal capable of generating a plasmonic band gap in optional energy by determining an arrangement period of the plasmonic crystal under given conditions.
A Surface Plasmon Polariton or SPP is a compressional wave of electrons existing at an interface between a metal and a dielectric, which is a surface electromagnetic wave propagating at the metal-dielectric interface. In the case that there is a periodic lattice structure on a metal surface of the metal-dielectric interface, the SPP is Bragg-reflected by the lattice in a certain wavelength and is propagated in the opposite direction to generate a standing wave. The lattice structure is referred to as a concavo-convex structure or a relief structure.
In the SPP which becomes the standing wave, electric fields are localized at anti-nodes thereof. The SPP which has become the standing wave generates two different energy states by relative phases with respect to the lattice. As a result, an energy gap referred to as a plasmonic band gap appears at a dispersion curve of the SPP.
The periodic relief structure on the metal surface which generates the plasmonic band gap is referred to as a plasmonic crystal. The plasmonic crystal has been researched from the basics to the applications because the control of a radiation field or enormous enhancement of the electric field can be expected. In “Applied Physics Letters, 2004, Vol. 85, p 3968-3970” (Non-patent document 1), an approach for enhancing light emitting intensity of dye by depositing dye thin films on a plasmonic crystal is reported. The enhancement technology in the emitting of dye using localization and enhancement of the electric field at a plasmomic band gap end (hereinafter, referred to as a merely gap end) is referred to as a plasmonic band gap laser. It is expected that the plasmonic band gap laser leads to development of an organic EL element and the like which can obtain high light emitting efficiency.
On the other hand, a sensor utilizing the SPP is used for detecting interaction among various materials in recent years. The sensor is referred to as a surface plasmon resonance (SPR) sensor.
When polarized light is incident on the metal-dielectric interface under the total reflection condition in order to excite the SPP, a bleeding component which is called an evanescent wave is generated at the interface. In the total reflection condition, energy of incident light will normally be energy of reflection light almost without loss. However, under a wavenumber matching condition in which the evanescent wave corresponds to the wavenumber of the SPP, energy of incident light is used for excitation of the SPP and the SPR is generated, which reduces energy of reflected light. The energy change of reflected light can be detected as change of wavenumber of reflected light with respect to incident light.
Since the wavenumber matching condition depends on an incident angle θ of incident light, when the wavenumber of reflected light is detected while changing the incident angle θ, the SPR is generated at a certain angle and the change of wavenumber will be the maximum. This angle is called resonance angle θsp, and the wavenumber of reflected light at the resonance angle θsp is called a peak wavenumber (or absorption peak).
Since the resonance angel θsp depends on permittivity of a dielectric, change of the resonance angle θsp and the peak wavenumber (hereinafter, referred to as a shift) corresponds to change of the permittivity of the dielectric. Therefore, the shift of the peak wavenumber is detected by using a sample including various materials as dielectrics, thereby detecting permittivity change of the sample caused by the interaction among materials.
In the SPR sensor, the interaction among materials in the sample is detected by the change of permittivity, therefore, it is not necessary to label materials by radioactive materials, fluorescent materials and the like, which enables sensitive detection. Accordingly, the SPR sensor is suitably used particularly as a bio sensor which evaluates the interaction of biological materials such as protein or nucleic acid. In Japanese Patent No. 3294605 (Patent Document 1), the SPR sensor as an optical bio-sensor device is disclosed.
In the SPR sensor, the change of permittivity of the sample, that is, the interaction among materials in the sample is detected by detecting the shift of the peak wavenumber. However, when detecting the interaction among materials whose molecular masses are extremely small or extremely minute amounts of materials, the shift of the peak number is less than a detection limit because the change of permittivity is small, as a result, it is sometimes difficult to obtain sufficient measurement sensitivity.
Thus, it is desirable to provide a design method of a plasmonic crystal which can be a basic technology for allowing the SPR sensor to be highly sensitive.